The present invention relates to a zero crossing detection circuit which is capable of operation with AC input signals having a wide range of voltage levels.
In a co-pending application entitled "VEHICLE DETECTOR WITH POWER MAIN NOISE COMPENSATION" by E. B. Hoekman and S. M. Hamer, which is assigned to the same assignee as the present application and filed on even date, a vehicle detector for a traffic control system is described. The vehicle detector compensates for noise produced by electric power distribution lines near an inductive sensor which can introduce a fluctuation in the frequency of the oscillator used by the vehicle detector to sense presence of a vehicle. In order to compensate for this fluctuation, the vehicle detector of the co-pending application senses phase of a power main signal during the time when a measurement is being made. The measurement is then compensated based upon the measured frequency of the oscillator signal, the phase of the power main signal, and a known relationship between measured frequency and the phase of the power main signal.
In the preferred embodiment shown in the co-pending application, phase of the power main signal is determined by sensing a positive going zero crossing of a power main signal. Since the frequency of the power main signal is known by measuring time between adjacent zero crossings of the same polarity, a time measurement from the positive-going zero crossing provides an indication of the phase of the power main signal.
Although zero crossing detector circuits have been known in the past, the application of a zero crossing detection circuit to a vehicle detector presents unique problems. In particular, the vehicle detector may be used anywhere in the world, and therefore the power main signal may be derived from a wide variety of different power systems. There is no single consistently available AC voltage or frequency. In addition, the available power main signal may have been stepped up or stepped down in voltage by the use of transformer. It is necessary, therefore, that the zero crossing detector circuit used in such an environment be capable of operating with an AC input signal which varies from as low as 3 volts AC to as high as 270 volts AC. In addition, the circuit must be capable of operating at various frequencies, including both 50 and 60 Hz. The zero crossing detection circuit must also dissipate only a small amount of power, so that it does not generate heat which would otherwise disrupt the operation of other components of the vehicle detector.